This invention relates to couplings for joining a driving shaft and a driven shaft which are approximately aligned on a central axis. More particularly, this invention is a flexible annular coupling made up of at least two members in the form of sectors of an annulus.
A flexible elastomeric coupling connects the shafts of rotating equipment. It is primarily made up of a center flexible elastomeric member bonded to a steel shoe on each axial end. The elastomeric member is usually made of a synthetic material, such as rubber, urethane, or their derivatives. Connection between the shafts is established by attaching the steel shoes of the coupling to hubs mounted on the shafts. The main requirement of a coupling is to transmit power from the driving shaft to the driven shaft, smoothly and efficiently, with minimum loss and fluctuations. It should also be capable of accommodating different loads and shaft misalignments produced by the equipment it is connected to, and at the same time, provide enough damping to absorb fluctuations in loads. Some of the major loads that have been identified are-torsional, axial and radial loads. Torsional loads have two components- impulse or shock loads, produced during startup of the equipment, and steady state loads, produced when the equipment is running. These loads are dynamic in nature and vary with the type of equipment being used and the magnitude of misalignment between the shafts connecting the coupling. Axial and radial loads are normally caused due to misalignment between shafts connecting the coupling, and produce (1) large vibratory forces during startup and running/loading of the equipment (2) fatigue loads on the elastomeric element, due to repeated radial and axial flexing. High tensile and compressive forces, acting at different angles, are produced in the elastomer.
When the shaft is in motion, a combination of the above dynamic loads act on the coupling, causing the elastomeric element to flex. The amount of flex is proportional to the magnitude of the forces acting on the coupling. Since the behavior of the elastomeric material is non-linear in nature, for any given cross-section of the elastomer in a three dimensional plane, the amount of flex varies with the geometry of the elastomer and the direction of shaft forces. The flexing of the elastomer causes three dimensional shear, tensile and compressive stresses which may be either distributed or localized across the cross-section, at different planes, depending on the geometry. An ideal coupling would be one in which the stresses are distributed uniformly across the cross-section of the elastomer. Localized stress zones tend to magnify the stresses and are sensitive to load fluctuations caused by torque and misalignment. The elastomer in these areas is constantly subject to repeated tensile and compressive stresses, much higher than the average stress values, causing it to fail prematurely.
It has been observed that the behavior of the elastomeric material changes significantly with the change in geometry and cross-section, and is non-linear in nature. The coupling is usually made up of two halves and is therefore discontinuous. High shear stress zones are caused across the circumferential end faces of the elastomer. Also, high peel stresses are caused at the bond line which are proportional to the applied torque. Using thicker elastomer cross-section to offset the high shear stress zones does not always give better results, but on the contrary, may form localized high stress zones, causing the material to fail prematurely, or cause higher peel stresses in the bond line, causing bond failure. Also, it has been observed that for most cross-sections the stresses in the elastomer tend to peak near the shoe area of the coupling, the maximum concentration being just slightly above the shoe, decreasing gradually as you move away from the shoe-elastomer interface. These stresses vary with the geometry of the elastomer, especially near the shoe area, and are mainly caused due to unequal stretching of the elastomer, stretch/stress propagation between the elastomer and steel at the shoe area is very low and uneven. This causes high stress zones in the elastomer and at the bond line resulting in fatigue and bond failures. Away from the shoe area, the stretch propagation across the elastomer is more or less uniform and the stress distribution is somewhat normal, depending upon the geometry of the elastomer.
U.S. Pat. No. 4,634,400 dated Jan. 6, 1987 in the name of Niel W. Butzow, et. al. shows and describes a flexible coupling made of two halves with the circumferential end faces made with a particular structure in an attempt to distribute the stress evenly across the cross-section, and away from the shoe. Nevertheless, the coupling described by the Butzow, et. al. patent develops high stress concentrations at the bond line and certain areas of the elastomer above the bond line, These stresses cause the coupling to fail, either due to fatigue in the elastomer or failure of the bond.
The foregoing illustrates limitations known to exist in present devices and methods.
Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.